1. Field of the Invention
This invention relates to compositions for electrodes of nonaqueous electrolyte secondary batteries, electrodes for nonaqueous electrolyte secondary batteries using the same and nonaqueous electrolyte secondary batteries.
2. Description of Related Arts
In recent years, size and weight reduction of mobile information terminals including cellular phones, notebook computers and personal data assistants (PDAs) has rapidly progressed. Along with this, there has been an increasing demand to increase the capacity of batteries used as their driving power sources. In addition, the use of nonaqueous electrolyte secondary batteries has been expanded to applications requiring high power, such as hybrid electric vehicles (HEVs) and electric tools. Thus, the development of nonaqueous electrolyte secondary batteries is being polarized into two directions, i.e., the direction to increase capacity and the direction to increase power.
For the purpose of increasing the battery capacity, high-capacity positive electrode materials to replace lithium cobaltate and high-capacity negative electrode materials to replace graphite have been developed. However, positive and negative electrodes using lithium cobaltate and graphite, respectively, which are mainstream materials for current lithium secondary batteries, have excellent performance balance, and the operations of a wide variety of mobile devices have been designed to adapt to the characteristics of batteries using these materials. Therefore, at present, the development of high-capacity electrode materials to replace lithium cobaltate and graphite has not progressed so far. Particularly, a change in the type of negative electrode material causes a significant change in the charge-discharge curve of the battery and in turn a significant change in the battery operating voltage. For this reason, the replacement of graphite with other high-capacity negative electrode materials is difficult to advance under present circumstances.
Nevertheless, power consumption of mobile devices is increasing year by year, and there has been a strong demand to increase the capacity of batteries. Therefore, at present, in order to respond to the demand to increase the battery capacity, substantially no choice exists but to increase the packing density of the negative electrode using graphite or increase the thickness of the negative electrode mixture layer.
Meanwhile, for example, from the standpoint of reduction in environmental burden in producing nonaqueous electrolyte secondary batteries, there has been recently proposed a technique in which an aqueous slurry is used to produce a negative electrode. Known aqueous slurries used to produce negative electrodes include those using a latex-based binder, such as styrene-butadiene rubber (SBR). However, aqueous slurries using a latex-based binder are difficult to coat in a thick film. Therefore, as disclosed in, for example, Published Japanese Patent Application No. 2002-175807, a thickener, such as carboxymethyl cellulose (CMC), is generally added to such an aqueous slurry using a latex-based binder.
Aqueous slurries using CMC and a latex-based binder have excellent coatability, and the use of such an aqueous slurry facilitates the coating of a thick film. Therefore, a thick mixture layer can be formed in a single coating pass.
However, with the use of an aqueous slurry using CMC and a latex-based binder, there are attendant difficulties in achieving high adhesion strength between the current collector and the mixture layer.
In the present invention, as described hereinafter, a vinylpyrrolidone-based polymer is used as a binder resin. On the other hand, in Published Japanese Patent Application No. H10-106542, polyvinylpyrrolidone (PVP) is used as a thickener, and a slurry containing polyvinylpyrrolidone and montmorillonite mixed thereinto is used to produce an electrode. In Published Japanese Patent Application No. H09-213306, a mixture of polyvinyl acetate and PVP is used as a binder resin. However, these patent literatures do not disclose at all particular vinylpyrrolidone-based polymers as described in the present invention and operations and effects caused by using the same.